This invention deals generally with electric lamp and discharge devices, and more specifically with the structure of a faceplate upon which is constructed the photocathode of a multiple anode photomultiplier tube.
Photomultiplier tubes have become commonly used instruments for detecting low radiation levels. Typically, they consist of a glass envelope with an electron emitting photocathode located on the inside surface of a faceplate of the envelope. When light or other radiation strikes the faceplate, it is transmitted through the faceplate to the photocathode. Electrons emitted from photocathode are then directed toward and collected by an electron multiplier. The electron multiplier consists of several secondary electron emitting dynodes, the first of which receives the electrons from the photocathode. The several dynodes are usually located in a single grouping, frequently referred to as a dynode cage. The electron multiplier delivers its electrons to an anode which has an electrical output which is directly related to the quantity of electrons collected by the first dynode. Multiple section photomultiplier tubes are not all that uncommon. They are particularly useful in radiation studies, including the study of light sources, in which the radiation falls on a large area, with different intensities, time sequences, or patterns impinging upon various portions of the area irradiated. While such fields can be studied by arrays of individual photomultiplier tubes when the radiation field is large enough, for small fields it is extremely difficult to construct tubes small enough and to pack individual tubes close enough to attain good definition and to avoid blocking out significant regions with the external envelopes of the adjacent tubes.
Multiple section photomultiplier tubes alleviate this problem by furnishing the effect of several tubes in one envelope. This permits closer packing of the active elements because the adjacent sections are not separated by portions of two envelopes. Several multiple section photomultiplier tubes are now available and are covered in the prior art, but they have problems which are not associated with the use of multiple independent tubes.
One problem is the need toconstruct and physically locate the multiple sections within a small envelope. One solution to this problem has been to construct similar electron multiplier dynode cages for each of the several sections, to locate them in close proximity to each other and then to attempt to isolate them in terms of the electron optics of the tube sections, so that the sections will operate independently. This has not always been successful.
"Crosstalk", that is, the interchange of signals between tube sections, is a continuing source of problems in such tubes, and many designs have been proposed to counteract such crosstalk. Crosstalk can occur not only between the electrons generated by the several dynodes, when the electrons move between electron multiplier sections, but also in the faceplate of the tube between the outside glass surface and the photocathode. In this situation, light falling on one section of the faceplate is transmitted across the fafaceplate thickness at some angle so that it actually affects a photocathode associated with a different section of the tube, thus yielding false information about the location of light falling on the faceplate.
One solution to this optical crosstalk in the region of the faceplate between the outside of the tube and the photocathode has been to place light shields within the faceplate to isolate the light falling on each section of the faceplate from the other sections.
Such optical isolation has until now been accomplished in some photomultiplier by inserting metal shields into grooves in the outside surface of the faceplate, but this structure has not fully isolated faceplate sections because such slots must not, of course, penetrate the entire thickness of the faceplate, or they would compromise the vacuum within the tube. When such metal strips penetrate the entire thickness of the faceplate and are fully integrated into the faceplate, it requires a complex grid structure of individual small glass faceplates with multiple glass to metal seals to the isolating metal grid. Such structures are very costly to construct and yield poor reliability due to the many possibilities of vacuum leaks.
Another structure which has been used for optical isolation, but only in single section image intensifier tubes, has been opaque glass fused around the one clear glass section of the faceplate. While such structures are reasonable for single section faceplates, they would be extremely complex and quite difficult to construct in a multiple section faceplate. In effect, each section of the faceplate would have to be individually constructed and then the several sections would have to be joined. Such a structure would likely be even more difficult to construct and less reliable than the multiple section faceplate built from a metal grid isolating structure sealed to individual clear glass faceplates.